Speed, efficiency, and accuracy, among other criteria, are manufacturing floor benchmarks. In an ever increasingly competitive manufacturing market, the firm that can fabricate higher quality parts in less time with a more efficient utilization of resources will succeed versus its less capable counterparts. One such industry is the semiconductor industry. Engineers in the semiconductor industry, particularly those involved with wafer and die processing and control equipment, have many devices from which to choose for motion control apparatuses. One such device is a voice coil motor, also called a voice coil actuator.
Originally used in loudspeakers, voice coil motors are limited motion devices that utilize a coil within a permanent magnetic field to produce a mechanical force proportional to the current applied to the coil. The motors are either linear or rotary motion, the latter of which generally has performance measured in torque versus force. Characteristic of either orientation is high acceleration (e.g., 50 gravities or more) and high frequency actuation.
The voice coil motors can be controlled by either a closed loop or open loop control system. In a closed loop system, a position sensor can provide feedback to the motor drive electronics to control the actuation of the voice coil motor. In the open loop system the actuation of the voice coil motor is controlled simply by altering the current applied to the coil, where it is generally known that, for example, a particular current results in a particular actuation, force, or torque. For either arrangement, uniformity and controllability are important.
Voice coil motors in the semiconductor industry frequently appear to control z-axis (i.e., normal to the plane containing the surface of the semiconductor wafer) in wirebond machines. Operating in a manner similar to a sewing machine, a wirebond machine connects integrated circuit die between and among other integrated circuits, circuit elements, and circuit substrate features with very fine conducting wires (e.g., generally 0.001 inch diameter or less).
As semiconductor device sizes decrease and device density increases, it is becoming more important that the wirebond machines are simultaneously highly position accurate and fast. Further, as the layers onto which the wirebond machine adheres wire interconnects become thinner and often more fragile, the z-axis control is critical so as to properly adhere, through for example applied force, ultrasonic vibration, and heat, the wire to the semiconductor device without damaging the surface of the device. The controllability, speed, and efficiency, among other features, (i.e. “performance”) of the voice coil motor depends in part on the configuration of the permanent magnets and resulting permanent magnetic field in which the coil operates.